1. Field
The present invention relates generally to communication, and more specifically to techniques for supporting successive interference cancellation (SIC) receiver processing with selection diversity in a multiple-input multiple-output (MIMO) communication system.
2. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
For a full-rank MIMO channel, with NS=NT≦NR, a transmitter may process (e.g., encode, interleave, and modulate) NT data streams to obtain NT symbol streams, which are then transmitted from the NT transmit antennas. The transmitted symbol streams may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different received signal-to-noise ratios (SNRs). Moreover, due to scattering in the communication link, the transmitted symbol streams interfere with each other at a receiver.
The receiver receives the NT transmitted symbol streams via NR receive antennas. The receiver may employ a successive interference cancellation (SIC) processing technique to process the NR received symbol streams from the NR receive antennas to recover the NT transmitted symbol streams. A SIC receiver processes the received symbol streams in NT successive stages to recover one transmitted symbol stream in each stage. For each stage, the SIC receiver initially performs spatial or space-time processing on the received symbol streams to obtain “detected” symbol streams, which are estimates of the transmitted symbol streams. One of the detected symbol streams is selected for recovery. The receiver then processes (e.g., demodulates, deinterleaves, and decodes) this detected symbol stream to obtain a decoded data stream, which is an estimate of the data stream for the symbol stream being recovered.
Each “recovered” symbol stream (i.e., each detected symbol stream that is processed to recover the transmitted data stream) is associated with a particular “post-detection” SNR, which is the SNR achieved after the spatial or space-time processing at the receiver. With SIC processing, the post-detection SNR of each recovered symbol stream is dependent on that stream's received SNR and the particular stage in which the symbol stream is recovered. In general, the post-detection SNR progressively improves for later stages because the interference from symbol streams recovered in prior stages is canceled (assuming that the interference cancellation is effectively performed).
The NT transmit antennas are associated with NT post-detection SNRs achieved by the NT symbol streams sent from these antennas. These NT post-detection SNRs are obtained for a specific ordering of recovering the NT symbol streams at the receiver. It can be shown that there are NT! possible orderings of recovering the NT symbol streams and thus NT! possible sets of post-detection SNRs, where “!” denotes a factorial. The receiver may evaluate all NT! possible orderings and select the ordering that provides the best set of post-detection SNRs.
The post-detection SNR of a transmit antenna determines its transmission capacity. Depending on the channel conditions, the post-detection SNR of a given transmit antenna may be so low that it cannot support the lowest data rate for the MIMO system. In this case, it may be beneficial to turn off that transmit antenna and only use the remaining transmit antennas for data transmission. Turning off a transmit antenna that cannot support the lowest data rate eliminates a symbol stream that would otherwise have interfered with the other symbol streams. This may then improve the post-detection SNRs of the other symbol streams.
Selection diversity refers to using only transmit antennas that can support at least the lowest data rate and turning off transmit antennas that cannot support the lowest data rate. If each transmit antenna can be turned on or off independently, then it can be shown that there are
      N    total    =            (                        N          T                !            )        ·          (              1        +                  1                      1            !                          +                  1                      2            !                          +                  …          ⁢                                          ⁢                      1                                          (                                                      N                    T                                    -                  1                                )                            !                                          )      possible orderings to evaluate. For example, if NT=4, then there are NT!=24 possible orderings without selection diversity whereby all NT transmit antennas are used, and Ntotal=64 possible orderings with selection diversity whereby each transmit antenna may be turned on or off independently. This represents a large increase in the number of orderings that the receiver may need to evaluate for selection diversity.
There is therefore a need in the art for techniques to support SIC receiver processing with selection diversity without the need to evaluate all Ntotal possible orderings.